(a) Technical Field
The present invention relates to a system and method for recovering performance of a fuel cell stack. More particularly, the present invention relates to a system and method for recovering performance of a degraded polymer electrolyte fuel cell stack through electrode reversal.
(b) Background Art
A fuel cell stack is a type of generating device that generates electricity as main energy source of a fuel cell vehicle and is configured in such a manner that several tens to several hundreds of unit cells are stacked. The configuration of a unit cell of the fuel cell stack will be described with reference to FIG. 5. A membrane electrolyte assembly (MEA) is positioned in the center of the unit cell.
The membrane electrolyte assembly includes a polymer electrolyte membrane 10 (capable of transporting hydrogen ions (protons)), a cathode 12 and an anode 14. The cathode and anode are stacked on both sides of the electrolyte membrane 10 so that hydrogen and oxygen react with each other. Here, both the anode and the cathode each include an electrode layer (Pt/C) in which platinum (Pt) is supported on carbon.
Moreover, although not shown in FIG. 5, a gas diffusion layer (GDL) is stacked on the outside of each of the cathode 12 and the anode 14, a separator in which flow fields are formed to supply fuel and discharge water produced by a reaction is stacked on the outside of the gas diffusion layer, and an end plate for supporting and fixing a plurality of unit cells is connected to the outermost side end of the fuel cell.
Accordingly, at the anode of the fuel cell stack, an oxidation reaction of hydrogen occurs to produce hydrogen ions (protons) and electrons, and the produced hydrogen ions and electrons are transmitted to the cathode through the polymer electrolyte membrane and the separator. At the cathode, the hydrogen ions and electrons transmitted from the anode react with the oxygen-containing air to produce water. At the same time, electrical energy is generated by the flow of electrons.
During fuel cell stack operation, the polymer electrolyte membrane, which makes up the membrane electrode assembly, and the cathode and the anode, i.e., the electrode layers (Pt/C), stacked on both sides of the polymer electrolyte membrane become degraded, and the performance of the fuel cell stack is reduced as a result of degradation after a certain period of operation.
In particular, it is known that oxide films (i.e., Pt-oxide such as Pt—OH, Pt—O, Pt—O2, etc.) formed, due to the degradation, on the surface of platinum of the cathode, having a particle size of several nanometers, interfere with the adsorption of oxygen (O2) onto the surface of platinum to reduce the rate of an oxygen reduction reaction (ORR) at the cathode, thus degrading the performance of the fuel cell stack. Moreover, carbon monoxide (CO) of several parts per million contained in the fuel (hydrogen) is chemically adsorbed onto platinum to decrease the efficiency of hydrogen oxidation reaction (HOR). Furthermore, it is known that a local temperature increase of the fuel cell stack occurring during high power operation of the fuel cell vehicle shrinks the pore structure of the electrolyte membrane or rearranges SO3− terminal groups to reduce ionic conductivity.
However, the performance degradation due to the structural changes in the membrane electrode assembly, i.e., the platinum oxide films, the CO in the fuel, the reduction in pores of the electrolyte membrane, etc., is mostly considered an irreversible degradation, and thus a method for recovering the membrane electrode assembly is needed.
One method for recovering performance of a degraded fuel cell stack includes supplying hydrogen to a cathode of the degraded fuel cell stack and storing the fuel cell stack for a predetermined time. An oxide formed on the surface of a platinum catalyst of the cathode is then reduced and removed while the fuel cell stack is stored for a predetermined time. These steps are then repeated a number of times to reduce the oxide on the surface of the platinum catalyst of the cathode.
As shown in FIG. 5, when hydrogen is supplied at 70° C. to the cathode 12 of the degraded fuel cell stack for more than 1 hour and the fuel cell stack is stored for 1 day at least three times repetitively so that the oxide films (PtOH, PtO, etc.) formed on the surface of platinum catalyst of the cathode 12 are removed and, at the same time, mobile platinum ions (Ptx+, x=2,4), which are released during operation of the fuel cell stack, combine with electrons and are re-precipitated as highly active platinum (Pt) to recover the catalytic properties of the cathode, the performance of the fuel cell stack can be recovered by about 30 to 40%.
Moreover, the hydrogen supplied to the cathode 12 for 1 hour is diffused back to the anode 14 (expressed by the dotted arrow in FIG. 5), which consequentially forms a hydrogen atmosphere in both electrodes, thus reducing the catalyst oxides of the cathode.
However, the above-described method for recovering the performance of the fuel cell has drawbacks in that it takes too long time to recover the performance and the amount of hydrogen supplied to the cathode is too large. Thus, due to these problems with the system, it is very difficult to effectively recover performance without removing the fuel cell stack from the fuel cell vehicle.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.